Exactly 16 years and 90 minutes after his birth, he received his driver's license. During the time I spent waiting at the Department of Motor Vehicles, only slightly shorter and less painful than my entire six-hour labor, I marveled at how quickly he had grown from 20 inches to 70 inches; from 7 pounds, 5 ounces to 150 pounds; from dependent to independent.

On Jan. 25, my husband, Larry, turns 50.

Jewish tradition tells us that 50 is the age for giving advice, according to Pirke Avot. This is, of course, redundant for someone who has been practicing law for half his life. And for someone who, I am confident, was born wise and compassionate. I'm only grateful I don't have to pay his hourly fee every time he offers counsel.

And sundown on Jan. 21, all the trees become one year older.

Yes, trees.

Judaism, which traditionally eschews birthday celebrations of humans, preferring instead to commemorate the anniversary of a person's death, marks Tu B'Shevat, the 15th of the month of Shevat, as the birthday of the trees.

This date, first designated in the Mishnah (Rosh Hashanah 1:1), determines when to start counting the age of a tree. This was important in ancient society as it was forbidden (Leviticus 19:23) to pick fruit from a tree less than 3 years old. Of course, according to this method of computation, a tree planted on the 14th of Shevat aged a year in only a day.

Additionally, Tu B'Shevat marked the start of the arboreal fiscal year. Farmers were compelled to give a tenth of their fruit as a tax, or tithe, to support the priesthood and the poor. Thus, fruit picked up until that date was liable for that year's taxes.

While January or February constitutes winter in much of the world, the 15th of Shevat was selected because, in Israel's agricultural cycle, the trees have soaked in enough water to nourish themselves by their own rising sap. Perhaps the sap equaled the groundhog's absent shadow, alerting everyone to the welcome advent of spring.

Over the years, while always a minor holiday, Tu B'Shevat has taken on different meanings and engendered different kinds of celebrations.

The 16th-century kabbalists in Sefad imbued the fruits with mystical qualities and celebrated with a Tu B'Shevat seder, which included a variety of fruits and four cups of wine, and which is increasingly popular today.

The early Zionists focused on reclaiming the land in Palestine, giving rise to the founding of the Jewish National Fund (JNF) in 1901. Since then, planting trees or donating to the JNF has become a regular part of Tu B'Shevat observance.

The environmental activists of the present, through organizations such as the Coalition on the Environment and Jewish Life, founded in 1993, have transformed the holiday into a Jewish Earth Day.

But consistently over the centuries, Tu B'Shevat has symbolized our vital and visceral connection to Eretz Israel. It has also reaffirmed our connection to all the earth, which belongs not to man but to God, and whose protection takes priority even in the time of war. Deuteronomy 20:19 confirms this: "When you besiege a city a long time, in making war against it, do not destroy the trees."

"Observe times and seasons," the Apocrypha tells us.

And we do, as a community. From the weekly celebrations of Shabbat, to the monthly rotations of the moon, to the yearly cycles of the holidays and festivals.

And on a personal level, as families and individuals, we observe brit milah and bar mitzvahs, graduations and confirmations, loves and losses.

And we celebrate birthdays -- to revisit our roots, to reaffirm our hopes and dreams and to ponder our progress. Birthdays also mark our place in the world's onrushing and unfathomable flow of time.

"Enjoy him now," strangers would say to me when Zack was young. "They grow so fast."

"Yes," I would answer, "and sometimes not fast enough."

But this month I am mystified and misty-eyed over my son's 16th birthday. And, with my husband's 50th, I am feeling solidly enmeshed in middle age.

Birthdays -- both for people and for trees -- are a time for reflection, renewal and, in our family, contributions to the JNF. After all, as Deuteronomy 20:19 points out, "A human is like a tree in the field.''

Jane Ulman lives in Encino, with her husband and four sons.

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